1. Field of the Invention
The invention is in the field of coating instruments with material supply; specifically, syringes for expressing bone cement form a self-contained reservoir.
2. Description of the Prior Art
Many prostheses are attached to large bones with a stem, which requires to be cemented into a bone; for example in the case of an artificial hip, into the proximal end of the femur. Since the prosthesis is made of strong materials, the bond of the prosthesis to the bone is the critical point of the installation. It is essential that the bond be as strong and as permanent as possible.
Bone cements are typically polymethylmethacrylate ("PMMA") compositions. A typical proprietary bone cement is Howmedica Radiopaque Simplex P (TM) made and sold by Howmedica Corporation. The cement comes in two components, powder and liquid monomer. The physician mixes the two shortly before use to form a pourable liquid, which is loaded into a syringe made for the purpose. The liquid rapidly thickens into a viscous paste, requiring considerable force for ejection from the syringe. The syringe is put into a hand-held ratchet "gun", whereby the viscous paste can be forced out of the syringe into the bone. U.S. Pat. No. 4,405,249 to Scales ably describes the operation of the syringe and the ratchet gun.
During the operation of mixing the cement components and filling the syringe, bubbles of air are inevitably entrained in the liquid; when the liquid thickens, the bubbles cannot escape from the paste. The bubbles of air are expressed with the cement into the bone; and when the cement hardens, the bubbles leave voids in the solidified cement.
If the voids are larger than a millimeter, they will considerably weaken the cement, and the bond between the prosthesis and the bone into which the prosthesis must be affixed. On the other hand, it has been shown that voids smaller than a millimeter, well distributed in the cement, are beneficial in fatigue tests. The small voids act to limit the propagation of cracks within the cement bond. This is discussed in detail in "Slow Crack Growth in Acrylic Bone Cement", Journal of Biomedical Materials Research, 9, 423-439 (1975).
Attempts have been made to remove the bubbles before the cement is used. One such effort involved centrifuging the liquid mixture of components. Though centrifugal forces got rid of bubbles, they also had the effect of classifying solid grains in the solid component of the cement according to size and density, which is unacceptable (see D.W. Burke, E.I. Gates and W.H. Harris, "Improvement of tensile and fatigue properties of PMMA by centrifugation", p. 128, Transactions 30th Annual Orthopedic Research Society Meeting, Atlanta, Ga., February 1984.) Unfortunately one cannot apply vacuum to the liquid mixture to remove air bubbles, because the vacuum causes the liquid component of the cement to vaporize, thereby making more voids in the mixture. Vacuum also causes entrained air to expand, and the viscosity of the cement paste captures the expanded bubbles before they can escape.
An alternative approach to the problem of voids is to break the air bubbles up into small bubbles of a size which will not weaken the cement bond, but which will enhance the fatigue properties of the cement. It has been found that this end can be achieved by forcing the cement paste through screens of the proper size in the barrel of the cement syringe.
Filtering screens have been used with injection syringes to screen out impurities in injectable fluids. Exemplary of such devices is U.S. Pat. No. 4,127,131, to Vaillancourt. In this device a filter disk is forced by a retainer ring into a recess in a hub which attaches to the nipple of the syringe. This arrangement, and others like it in which filtration occurs after the fluid in the syringe has passed through the nipple of the syringe, will not serve for the purpose of this invention. The small area of the filter would offer such great resistance to the viscous paste of the bone cement, that the force required would probably rupture the filter. Moreover, a bubble in the nipple of the syringe would be dispersed only in the small volume of cement contained within the nipple, whereas what is sought is as great a dispersion of microbubbles as is possible.
What is needed therefore is filtering means within the barrel of the cement syringe, of large enough mesh to pass the largest particles in the solid component of the cement paste, yet small enough to form microbubbles and disperse them among a quantity of the cement.